Nonaqueous electrolyte type secondary battery with a film casing

ABSTRACT

A secondary battery including an electricity Generating element, which includes at least a positive electrode implemented by a lithium-manganese compound oxide, a negative electrode, an electrolyte and a separator, and films encasing the electricity generating element is disclosed. The secondary battery further includes a composition causing the electrolyte to react with water to thereby produce hydrogen ions, and a hydrogen ion uptaking agent so positioned as to contact the electrolyte existing in the battery. The battery is desirable in charge-discharge cycle characteristic, storage characteristic, and safety. In addition, the battery swells little despite a repeated charge-discharge cycle or storage.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a nonaqueous electrolyte typesecondary battery including an electricity generating elementaccommodated in a film casing and more particularly to a nonaqueouselectrolyte type secondary battery configured to well little.

[0002] A secondary battery of the type described is conventional andincludes an electricity generating element and a casing accommodatingthe electricity generating element. The casing is implemented by twoaluminum laminate films each consisting or aluminum foil and athermo-bonding resin film formed on the aluminum foil. Metallic leadsprotrude from the inside to the outside of the casing via the bondedportion of the casing. Today, there is an increasing demand for a thin,light battery configuration capable of implementing a thin, lightelectric apparatus. In this sense, the battery accommodated in the filmcasing is advantageous over a battery accommodated in a hard metalliccasing.

[0003] The battery with the film casing needs more strict safetyimplementations than the battery with the hard metallic casing. Forexample, the battery should undergo a minimum of deformation in contour.Of course, the battery with the film casing should have an abilitygenerally required of a battery, e.g., high energy density (highcharge-discharge capacity), a high cycle characteristic, and a storagecapacity characteristic despite self-discharge.

[0004] Lithium manganate is attracting increasing attention as one ofpositive electrode substances for a lithium ion secondary battery.Lithium manganate has a spinel structure represented by LiMn₂O₄ andfunctions as a 4V class, positive electrode substance in relation to aλ-MnO₂ composition. Lithium manganate with the spinel structure has atridimensional host structure different from a layer structureparticular to, e.g., LiCoO₂, so that most of stoichiometric capacityavailable therewith can be used. Lithium manganate is therefore expectedto have a desirable cycle characteristic.

[0005] Further, lithium manganate with the spinel structure allowslithium ions to be pulled out while maintaining its basic frame. Thiscompound therefore starts releasing oxygen at a higher temperature thancobalt acid lithium having a layer, halite structure and is expected tobe desirable from the safety standpoint. The safety feature isparticularly important when it comes to the battery with the soft filmcasing- In practice, however, a lithium secondary battery including apositive electrode implemented by lithium manganate has a problem thatits capacity decreases little by little due to repeated charging anddischarging. This problem is serious in the aspect of practical use.

[0006] Various schemes have heretofore been proposed to improve thecycle characteristic of an organic electrolyte type secondary batterywhose positive electrode is implemented by lithium manganate. Forexample, Japanese Patent Laid-Open Publication Nos. 3-67464, 3-119656,3-127453, 7-245106 and 7-73883 teach improvements achievable byimproving reactiveness at the time of production. Also, Japanese PatentLaid-Open Publication Nos. 4-198028, 5-28307, 6-295724 and 7-97216 teachimprovements achievable by controlling a grain size. Further, JapanesePatent Laid-Open Publication No. 5-21063 teaches an improvementattainable by removing impurities. None of such schemes, however,achieves a satisfactory cycle characteristic.

[0007] Japanese Patent Laid-Open Publication No. 2-270268 proposes toimprove the cycle characteristic by selecting an Li composition ratiosufficiently greater than a stoichiometric ratio. This kind of scheme isdisclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 4-123769,4-147573, 5-205744 and 7-282798 also. Experiments actually indicated theimprovement in cycle characteristic achievable with such a scheme.

[0008] Japanese Patent Laid-Open Publication Nos. 6-338320 and 7-262984,for example, each use LiMn₂O₄, which is a Mn spinel substance, andLi₂Mn₂O₄, LiMnO₂, Mi₂MnO₃ or similar Li-Mn compound oxide, which isricher than the above Mn spinel substance, as a positive electrodeactive substance. However, adding excessive Li or mixing it with anotherLi-rich compound reduces the charge-discharge capacity andcharge-discharge energy although improving the cycle characteristic. Asa result, high energy density and long cycle life are not compatiblewith each other. By contrast, Japanese Patent Laid-Open Publication No.6-275276 proposes to increase a specific surface area for achieving ahigh rate, charge-discharge characteristic (great current relative tocapacity at the time of charging and discharging) and perfect reaction.This, however, obstructs an increase in cycle life.

[0009] On the other hand, it has been studied to improve thecharacteristics by adding another element to a Li-Mn-O compound. Forexample, Japanese Patent Laid-Open Publication Nos. 4-141954, 4-160758,4-169076, 4-237970, 4-282560, 4-289662, 5-28991 and 7-14572 each proposeto add or dope, e.g., Co, Ni, Fe, Cr or Al. However, the addition ofsuch a metal element reduces the charge-discharge capacity and needsfurther studies to satisfy the total ability.

[0010] As for the addition of another element, boron is expected toimprove the other characteristics, e.g., cycle characteristic andself-discharge characteristic while degrading the charge-dischargecharacteristic little. This described in, e.g., Japanese PatentLaid-Open Publication Nos. 2-253560, 3-297058, and 9-115515. In anycase, a manganese dioxide or a lithium-manganese compound oxide is mixedwith a boron compound (e.g. boric acid) in a solid phase or immersed inan aqueous solution of a boron compound and then heated, therebyproducing a lithium-manganese compound oxide. The resulting compoundpowder of boron compound and manganese oxide decreases in surfaceactivity and is expected to suppress reaction with an electrolyte andtherefore to improve the storage characteristic.

[0011] The addition of boron, however, reduced the growth of particlesand tap density and did not directly translate into high capacityrequired of a battery alone. Moreover, capacity decreased in the validpotential range when boron was combined with a carbon negativeelectrode, or the reaction of boron with an electrolyte could not besufficiently suppressed, depending on the synthesizing conditions. Borontherefore did not satisfactorily improve the storage characteristic.

[0012] On the other hand, lithium manganate applied to the positiveelectrode of the battery with the film casing did not satisfy theexpected degree of safety. Specifically, when the battery was repeatedlycharged and discharged or left in a charging state at a hightemperature, gases were generated in the battery and raised the pressureinside the battery, causing the battery to easily swell. The above gasesare presumably ascribable to the decomposition of the electrolyte Theswell of the contour of the battery is apt to exceed a space allocatedthereto when mounted to an electric apparatus, exerting pressure onsurrounding parts. In the worst case, the gases bring about thedangerous burst of the battery.

[0013] As stated above, although lithium manganate is a hopeful compoundoxide capable of replacing LiCoO₂, which is the predominant positiveelectrode active substance, the conventional battery using lithiummanganate has the following problems (1) through (3) left unsolved.

[0014] (1) High energy density (high charge-discharge capacity) and highcycle life are not easily compatible.

[0015] (2) Storage capacity decreases due to self-discharge.

[0016] (3) When the battery with the film casing and using LiMn₂O₄ isrepeatedly charged and discharged or held in a charging state in a hightemperature environment, gases presumably ascribable to thedecomposition of the electrolyte are generated and cause the battery toswell.

[0017] Technical problems relating to the production of a battery andthe compatibility of lithium manganate with an electrolyte have beenpointed out as the causes of the above problems (1) through (3). Payingattention to the material of the positive electrode itself and theinfluence of the material, the above problems may be accounted for, aswill be described hereinafter.

[0018] (1) Compatibility of High Energy Density and High Cycle Life

[0019] As for the decrease in capacity ascribable to thecharge-discharge cycle, the mean valence of Mn ions varies betweentrivalence and tetravalence as charge compensation derived from theingress and egress of Li. As a result, Jahn-Teller distortion occurs incrystal. In addition, Mn is eluted from lithium manganate, or impedanceincreases as a result of the Mn elution. More specifically, the decreasein capacity ascribable to repeated charge-discharge cycle is broughtabout mainly by the following causes:

[0020] (a) influence of impurities;

[0021] (b) elution of Mn from lithium manganate and precipitation ofeluted Mn on a negative electrode active substance or on separator;

[0022] (c) inactivation ascribable to the isolation of active substanceparticles;

[0023] (d) influence of acids derived from contained water; and

[0024] (e) deterioration of an electrolyte ascribable to the release ofoxygen from lithium manganate.

[0025] Assume that a single spinal phase is formed. Then, Mn is elutedpresumably because trivalent Mn partly changes into tetravalent Mn anddivalent Mn and causes Mn to be easily dissolved in an electrolyte, andbecause the relative amount of Li ions becomes short. Presumably,repeated charging and discharging accelerates the generation ofirreversible capacity and disturbance to the atomic arrangement ofcrystal, while causing the eluted Mn ions to precipitate on the negativeelectrode or on the separator and obstruct the migration of Li ions.Furthermore, the cubic symmetry of lithium manganate is distorted due tothe ingress and egress of Li on the basis of the Jahn-Teller effect,resulting in the expansion or contraction of the unit lattice length byseveral percent. Consequently, the repeated charge-discharge cycle mayeven make electric contact partly defective or may prevent the isolatedparticles from functioning as an electrode active substance.

[0026] Furthermore, the elution of Mn presumably promotes the release ofoxygen from lithium manganate. Lithium manganate with many oxygendefects is increased in 3.3 V Plato capacity and therefore deterioratedin cycle characteristic as the cycle is repeated. Also, the release ofmuch oxygen presumably has adverse influence on the decomposition of anelectrolyte and thereby deteriorates the cycle. It has been proposed tosolve this problem by, e.g., improving the synthesizing method, addinganother transitional element or enriching the Li composition. None ofthese schemes has succeeded to achieve both of high discharge capacityand high cycle

[0027] In light of the above, there may be reduced the distortion of alattice or to reduce oxygen defects by way of example.

[0028] (2) Decrease in Storage Capacity due to Self-Discharge

[0029] Regarding this problem, let the defective alignment of positiveand negative electrodes ascribable to a production process and internalshorting derived from the entry of metal waste be excepted. Then, animprovement in storage characteristic is expected to enhance thestability of lithium manganate, i.e., to reduce Mn to be eluted,reaction with an electrolyte, and release of oxygen.

[0030] (3) Swell of Battery Using LiMn₂O₄

[0031] Presumably, the gases that cause the battery to swell areascribable to an occurrence that Mn is precipitated on a negativeelectrode active substance and forms a high-resistance film on thesurface or the substance. The high-resistance film is likely to promotethe decomposition of an electrolyte on the surface of the negativeelectrode, resulting in the generation of hydrogen gas. Further, oxygenreleased from lithium manganate may produce oxygen gas, carbon monoxidegas and carbon dioxide gas on the surface of a positive electrode. Suchgases cause the battery with the film casing to swell.

[0032] The above-described problems (1) through (3) become more seriousin a high-temperature environment, preventing the applicable range frombeing extended. However, only a limited range of materials are availablethat implement potential, which satisfies performance required of thestate-of-the-art high performance secondary battery, e.g., highelectromotive force, flat voltage during discharge, cyclecharacteristic, and energy density. It is therefore necessary to realizenew lithium manganate with spinel structure that obviates thedeterioration of capacity ascribable to charging and discharging and hasdesirable cycle characteristic and storage characteristic.

[0033] Japanese Patent Laid-Open Publication No. 10-112318 teaches thata positive electrode active substance is implemented by a mixture ofLiMn₂O₄ or similar lithium-manganese compound oxide and LiNiO₂or similarlithium-nickel compound oxide. In accordance with this document, greatcharge-discharge capacity is achievable because irreversible capacity atthe time of initial charging is made up for . . . Japanese PatentLaid-Open Publication No. 7-235291 also describes thatLiCo_(0.5)Ni_(0.5) O₂ is introduced into LiMn₂O₄ in order to synthesizea positive electrode active substance.

[0034] We, however, experimentally found that the mixture of thelithium-manganese compound oxide and the lithium-nickel compound oxidedid not satisfy the charge-discharge characteristic, particularly cyclelife and capacity storage at high temperatures, or self-dischargecharacteristic alone. This is because not all the lithium-nickelcompound oxides can uptake hydrogen ions, as will be describedspecifically later. That is, only a particular lithium-nickel compoundoxide in accordance with the present invention can effectively obviatethe deterioration of a lithium-manganese compound oxide or that of anelectrolyte.

[0035] Moreover, the mixture of the lithium-manganese compound oxide andthe lithium-nickel compound oxide was used as the positive electrodeactive agent of the battery including the film casing. The mixture,however, had no effect as to the swell of the battery ascribable to thecharge-discharge cycle and the storage in a charging state (particularlyat high temperatures) alone.

[0036] Technologies relating to the present invention are also disclosedin, e.g., Japanese Patent Laid-Open Publication Nos. 10-125323,11-071115, 11-204110 and 11-297361, Japanese Patent Nos. 2512239,2512241, 2517176, 2547137, 2584123, 2797526 and 2797528, and JapanesePatent Publication Nos. 7-70329, 7-73051 and 7- 118317.

SUMMARY OF THE INVENTION

[0037] It is therefore an object of the present invention to provide anonaqueous electrolyte type secondary battery with a film casingdesirable in battery characteristic, particularly a charge-dischargecharacteristic and a storage characteristic, and safety and swellinglittle despite a repeated charge-discharge cycle or storage.

[0038] A secondary battery of the present invention includes anelectricity generating element, which includes at least a positiveelectrode implemented by a lithium-manganese compound oxide, a negativeelectrode, an electrolyte and a separator, and films encasing theelectricity generating element is disclosed. The secondary batteryfurther includes a composition causing the electrolyte to react withwater to thereby produce hydrogen ions, and a hydrogen ion uptakingagent so positioned as to contact the electrolyte existing in thebattery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription taken with the accompanying drawings in which:

[0040]FIG. 1 is an isometric view showing a nonaqueous electrolyte typesecondary battery embodying the present invention;

[0041]FIG. 2 is a sectional view of the secondary battery shown in FIG.1;

[0042]FIG. 3 is a table listing the results of analysis of Mn ionconcentrations in an electrolyte;

[0043]FIG. 4 is a table listing the results of analysis of Li ionconcentrations in an electrolyte;

[0044]FIG. 5 is a table showing a relation between a cycle estimationtemperature and a #50/#1 residual capacity ratio;

[0045]FIG. 6 is a graph comparing a specific example of the illustrativeembodiment and a comparative example with respect to 55° C. dischargecapacity and a cycle characteristic;

[0046]FIG. 7 is a table showing a relation between specific examples ofthe illustrative embodiment and a comparative example with respect tocapacity determined by immediate discharge and capacity determined bydischarge after twenty-eight days;

[0047]FIG. 8 is a table comparing specific examples of the illustrativeembodiment and a comparative example with respect to safety;

[0048]FIG. 9 is a graph showing a relation between the specific surfacearea and the Mn elution of a lithium-nickel compound oxide;

[0049]FIG. 10 is a graph showing a relation between the specific surfacearea of a lithium-nickel compound oxide and the viscosity and conditionof a slurry and the coated condition of an electrode;

[0050]FIG. 11 is a table comparing a specific example of theillustrative embodiment and a comparative example with respect to achange in the thickness of a battery ascribable to a repeatedcharge-discharge cycle; and

[0051]FIG. 12 is a table comparing a specific example of theillustrative embodiment and a comparative example with respect to achange in the thickness of a battery ascribable to storage.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0052] The principle of the present invention will be described first.In accordance with the present invention, in a nonaqueous electrolytetype secondary battery including an electricity generating element,which includes at least a positive electrode implemented by alithium-manganese compound oxide, a negative electrode, an electrolyteand a separator, and films encasing the electricity generating element,a composition causing the electrolyte to easily react with water tothereby produce hydrogen ions is provided. A hydrogen ion uptaking agentis so positioned as to contact the electrolyte existing in the secondarybattery.

[0053] In accordance with the present invention, the electrolytecontains a supporting electrolyte that is either one of LiPF₆ and LiBF₄.

[0054] We reached the present invention as a result of extendedresearches and experiments directed toward the reduction of Mn to beeluted from the lithium-manganese compound oxide. Assume a nonaqueouselectrolyte type secondary battery using a lithium-manganese compoundoxide as a positive electrode active substance. Then, it is possible todetermine the deterioration of the cycle characteristic by using a Mnion concentration in an electrolyte as an index. Also, it is possible todetermine the deterioration of the storage characteristic on the basisof a change in a Li ion concentration in the electrolyte. Gases causingthe battery to swell are ascribable partly to the decomposition of thepositive electrode active agent (elution of Mn and release of oxygen)and partly to Mn precipitated on the surface of a negative electrode andcausing the electrolysis of the electrolyte. The generation of suchgases can therefore be determined by use of a Mn ion concentration inthe electrolyte.

[0055] We experimentally found that LiPF₆ or LiBF₄ used as the Lisupporting salt caused Mn ions to be eluted in the electrolyte in aparticularly great amount and increased the acidity of the electrolyte.Presumably, therefore, the supporting salt and a small amount of waterexisting in the organic electrolyte react with each other to generatehydrogen ions (H⁺). Such hydrogen ions are considered to elute Mncontained in the lithium-manganese compound oxide and therebydeteriorate the crystal structure.

[0056] We considered that by locating a compound capable of uptakinghydrogen ions at a position where it could contact the electrolyte, ahydrogen ion concentration in the electrolyte and therefore the elutionof Mn ions into the electrolyte could be reduced.

[0057] In fact, experiments showed that a hydrogen ion uptaking agentnoticeably reduced Mn ions to be eluted into the electrolyte andsuppressed a change in the Li ion concentration in the electrolyte.Further, the above agent successfully reduced the deterioration andcolor change of the electrolyte a well as the generation of acids andgases. The reduction in the amount of Mn ions to be eluted into theelectrolyte translates into the reduction of release of oxygen from thelithium-manganese compound oxide. This not only protects the crystalstructure of the compound oxide, but also obviates oxygen gas, carbonmonoxide gas and carbon dioxide gas.

[0058] Consequently, the present invention improves the cyclecharacteristic while preserving high charge-discharge capacity, preventsimpedance from increasing because of the reduction of decomposition ofthe electrolyte and a change in Li concentration, and obviates gases,which would cause the battery to swell.

[0059] Referring to FIGS. 1 and 2 of the drawings, a nonaqueouselectrolyte type secondary battery embodying the present invention isshown. As shown, the secondary battery includes a winding type ofelectricity generating element (simply generating element hereinafter)2. The generating element 2 is enclosed in a casing implemented bylaminate films 1 a and 1 b. The laminate films 1 a and 1 b each consistof a thermo-bonding resin film 11, metal foil 12, and heat-resistantresin film 13 The laminate films 1 a and 1 b are-positioned such thattheir thermo-bonding resin films 11 face each other. The generatingelement 2 is bonded to the laminate films 1 a and 1 b at its four edgesby heat with a positive lead 31 and a negative lead 32 protruding fromone side of the element 2. The laminate films 1 a and 1 b each aremolded in such a manner as to have a bottom and sides complementary tothe contour of the generating element 2. The generating element 2 isimpregnated with a nonaqueous electrolyte and includes a positiveelectrode, a negative electrode and a separator, not shown, as well asthe electrolyte.

[0060] A hydrogen ion uptaking agent applicable to the present inventionreacts with hydrogen ions (H⁺) present in an organic electrolyte andlowers hydrogen ion concentration. The uptaking agent should preferablychange, as a result of the reaction, into a compound or an inactivecompound that does not effect the battery system. A hydrogen ionuptaking agent that produces water as a result of the above reaction isnot feasible for the present invention because such water would producehydrogen ions by again reacting with a supporting electrolyte. An alkalimetal hydroxide, for example, is not desirable because OH⁻ ions reactwith hydrogen ions and produce water. A substance that increases theimpedance of the battery to an excessive degree as a result of thereaction is not desirable either.

[0061] The hydrogen ion uptaking agent may be positioned anywhere in thebattery so long as it contacts the electrolyte. For example, theuptaking agent may be mixed with or dissolved or dispersed in theelectrolyte or may even be mixed in an electrode. Specifically, if theuptaking agent is capable of functioning as the material of anelectrode, it may be mixed in a lithium-manganese compound oxide, whichconstitutes a positive electrode in the present invention. While theuptaking agent may be either one of an organic compound and an inorganiccompound, use may be made of, e.g., a lithium-nickel compound oxide,hydrogen-occluding alloy or carbon capable of occluding hydrogen. Thiskind or substance should preferably be implemented as powder so as to bemixed in the positive electrode or dispersed in the electrolyte.

[0062] The lithium-nickel compound oxide will be described hereinafteras a preferable hydrogen uptaking agent. The lithium-nickel compoundoxide applicable to the present invention should be capable of uptakinghydrogen ions. For example, lithium-nickel compound oxides taught in thepreviously mentioned Laid-Open Publication Nos. 10-12318 and 7-235291are unable to uptake hydrogen ions.

[0063] The lithium-nickel compound oxide consists of lithium, nickel andoxygen and may be any one of LiNiL₂, Li₂NiO₂, LiNi₂O₄, Li₂Ni₂O₄ and soforth with or without another element doped therein for thestabilization of the oxide, higher capacity or higher safety. Forexample, LiNiO₂ doped with another element is expressed asLiNi_(1-x)M_(x)O₂ (0<×≦0.5) where M denotes a doped metal element, whichis one or more metal elements selected from a group consisting of Co,Mn, Al, Fe, Cu and Sr. If desired, two more doping metal elements may beused so long as the sum of the composition ratios of the elements is x.Among the above compounds, LiNiO₂ and LiNi_(1-x)Co_(x)O₂ areparticularly desirable.

[0064] In the present invention, the Li/Ni ratio (Li/[Ni+M] ratio in thecase of LiNi_(1-x)M_(x)O₂) of the lithium-nickel compound oxide may beslightly deviated from its stoichiometric value. The scope of thepresent invention includes such a condition as well.

[0065] The lithium-nickel compound oxide applicable to the presentinvention has a specific surface area X of 0.3 or above, so that thelithium-manganese compound oxide or the electrolyte is effectivelyprotected from deterioration. Generally, the specific surface area is5.0 or below. When the lithium-nickel compound oxide is to be mixed inthe positive electrode, the specific surface area should preferably be3.0 or below in order to implement slurry that is easy to handle and tocoat the electrode. It is to be noted that the specific surface arearefers to a surface area for a unit powder weight (m²/g) and is measuredby a gas adsorption method.

[0066] Further, the lithium-nickel compound oxide applicable to thepresent invention may have a D₅₀ grain size of 40 μm or below. This isalso successful to protect the lithium-manganese compound oxide or theelectrolyte from deterioration. Generally, the D₅₀ grain size is 1 μm orabove. When the lithium-nickel compound oxide is to be mixed in thepositive electrode, the D₅₀ grain size should preferably be 3 μm orabove in order to implement slurry that is easy to handle and to coatthe electrode. The D₅₀ grain size corresponds to 50% of a weight sum andis measured by a laser beam diffusion method.

[0067] A specific procedure for producing the above lithium-nickelcompound oxide will be described hereinafter. First, lithium may beimplemented by lithium carbonate, lithium oxide, lithium nitride,lithium hydroxide or similar lithium compound. Also, nickel may beimplemented by, e.g., nickel hydroxide, nickel oxide or nickel nitride.Both of lithium and nickel should preferably be pulverized to a suitablegrain size. Particularly, the resulting nickel powder should preferablybe sieved to the D₅₀ grain size.

[0068] Subsequently, the pulverized lithium and nickel are sufficientlymixed in a Li/Ni ratio that implements the target composition ratio ofthe lithium-nickel compound oxide. The lithium and nickel mixture isthen sintered a 500° C. to 900° C. in the same manner as during theproduction of a lithium-manganese compound oxide. The sinteredlithium-nickel compound oxide should preferably be further sieved tohave the target surface area and target D₅₀ grain size.

[0069] The lithium-nickel compound oxide produced by the above procedureand capable of forming part of the positive electrode should preferablybe mixed with the lithium-manganese compound oxide. Alternatively, sucha compound oxide may be dispersed in the electrolyte.

[0070] While the hydrogen ion uptaking function of the lithium-nickelcompound oxide has not been clearly accounted for, hydrogen ions arepresumably substituted for lithium ions contained in the crystal of thecompound oxide.

[0071] Assume that the lithium-nickel compound oxide is mixed with thelithium-manganese compound oxide and applied to the positive electrode.Also, assume a relation:

[LiMn compound oxide]:[LiNi compound oxide]=100−a:a

[0072] Then, by selecting a greater than or equal to 3, it is possibleto reduce Mm to be eluded from the lithium-manganese compound oxide tothe electrolyte and therefore to improve the cycle characteristic andcapacity preservation characteristic. Further, if a is selected to besmaller than or equal to 45, an extremely safe nonaqueous electrolytetype secondary battery is achievable.

[0073] The lithium-manganese compound oxide applicable to the presentinvention as a positive electrode active substance will be describedhereinafter. The lithium-manganese compound oxide is an oxide consistingof lithium, manganese and oxygen and may be, e.g., LiMn₂O₄ or similarlithium manganate having a spinel structure, LiMn₂O₄ or LiMnO₂. Amongthem, LiMn₂O₄ or similar lithium manganate with a spinel structure isparticularly desirable. So long as the lithium-manganese compound oxidehas a spinel structure, the Li/Mn ratio may be deviated from 0.5 and maybe between 0.5 and 0.65, preferably between 0.51 and 0.6 or morepreferably between 0.53 and 0.58. Likewise, the lithium-manganesecompound oxide with a spinel Structure may have a [Lin+Mn]/[O] ratiodeviated from 0.75.

[0074] The lithium-manganese compound oxide has a weight mean grain sizeusually ranging from 5 μm to 30 μm in order to facilitate the productionof slurry, which implements the positive polarity, and to make thebattery reaction uniform.

[0075] A specific procedure for producing the lithium-manganese compoundoxide having the above composition will be described hereinafter. First,lithium (Li) may be implemented by, e.g., lithium carbonate, lithiumoxide, lithium nitride, lithium hydroxide or similar lithium compound.Manganese (Mm) may be implemented by a manganese compound, e.g.,electrolytic manganese dioxide (EMD), Mn₂O₃, Mn₃O₄, chemical manganesedioxide (CMD) or similar Mn oxide, or manganese carbonate, manganeseoxalate or the like. The combination of electrolytic manganese dioxideand lithium carbonate is preferable because it allows the Li/Mn ratio tobe easily guaranteed. Moreover, such a combination is preferable in theaspect of energy density for a unit volume dependent on a difference inbulk density, control over grain size, process and handling in the eventof mass production, generation of toxic substances, and cost.

[0076] Before mixing Li and Mn, which are starting substances, it ispreferable to pulverize each of Li and Mn to a suitable grain size. TheMn grain size is usually between 3 μm to 70 μm, preferably between 5 μmand 30 μm, while the Li grain size is usually 10 μm or below, preferably5 μm or below and most preferably 3 μm or below.

[0077] The reaction that produces the Li-Mn compound oxide proceeds onthe surface of the solid phase. Therefore, if Li and Mn are notsufficiently mixed or if the grain sizes are excessively large, theLi-Mn compound oxide with the target composition and target structuresometimes cannot be achieved. For example, assume that Li and Mnexpected to produce lithium manganate having a spinel structure are notsufficiently mixed together or that the grain sizes are excessivelylarge. Then, Mn₂O₃, Mn₃O₄, Li₂MnO₃, Li₂Mn₄O₉, Li₄Mn₅O₁₂ or similar phaseis sometimes produced, lowering the voltage or the energy density of theresulting battery. In this sense, the grain sizes stated above desirablypromote uniform reaction and thereby increase the contact area betweenLi and Mn. The powder may be controlled in grain size or may be shaped,if desired. Control over the grain sizes allows the Li-Mn compound oxidehaving the target grain size to be easily obtained.

[0078] Subsequently, Li and Mn with a molar ratio matching the targetcomposition of the Li-Mn compound oxide are sufficiently mixed togetherand then sintered in an oxygen atmosphere. For oxygen, use may be madeof pure oxygen or a mixture gas of oxygen and nitrogen, argon or similarinactive gas. At this instant, the oxygen partial pressure is about 50torr to about 760 torr.

[0079] While the sintering temperature for producing lithium manganatewith a spinel structure is usually between 400° C. and 1,000° C., it maybe suitably selected to implement the desired phase. Specifically, itthe sintering temperature is excessively high, Mn₂O₃, Li₂MnO₃ or similarunexpected phase is generated and makes the resulting battery voltage orthe energy density short. Conversely, if the sintering temperature isexcessively low, the relative oxygen concentration becomes excessivelyhigh or the powder density becomes excessively low This is undesirablefrom the high capacity standpoint. The sintering temperature shouldtherefore be between 600° C. and 900° C., most preferably between 700°C. and 850° C.

[0080] While the sintering time maybe suitably controlled, it is usuallybetween 6 hours and 100 hours, preferably between 12 hours and 48 hours.While the cooling rate may also be suitably controlled, it is preferablyabout 100° C. per hour at the final sintering step because rapid coolingis not desirable. The lithium-manganese compound oxide powder producedby such a procedure may be further sieved, as needed.

[0081] The positive electrode of the secondary battery of the presentinvention uses the above described lithium-manganese compound oxide withor without the hydrogen ion uptaking agent as an active substance. Theuptaking agent may further additionally contain LiCoO₂ or similarcompound generally known as a positive electrode, active substance.Further, the uptaking agent may contain Li₂CO₃ or similar additive for asafety purpose. A method of producing the positive electrode is open tochoice. In an exemplary method, the lithium-manganese compound oxidepowder and lithium-nickel compound oxide powder are mixed together(slurry) in a suitable dispersant, which dissolves a binder, togetherwith, e.g., a conducting agent and a binder. After the resulting slurryhas been coated on aluminum foil or similar collecting material and thendried, the collecting material is pressed or otherwise processed to forma film thereon.

[0082] The conducting agent is open to choice and may be implemented by,e.g., carbon black, acetylene black, natural or artificial graphite,carbon fibers or similar conventional substance. For the binder, use maybe made of polytetratluoroethylene (PTFE), polyvinylidene fluoride(PVDF) or similar conventional substances

[0083] The electrolyte applicable to the present invention will bedescribed hereinafter. The electrolyte, which the present inventionuses, consists of a nonaqueous solvent and a supporting electrolytedissolved therein. The solvent may be implemented by any one ofcarbonates, chlorinated hydrocarbon, ethers, ketones, nitriles and soforth. Preferably, the solvent consists of at least one of ethylenecarbonate (EC), propylene carbonates (PC), γ-butyllactone or similarsolvent whose dielectric constant is low, and at least one of diethylcarbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EGC),ester or similar solvent whose dielectric constant is high. The solventshould preferably be the combination of PC and MC or the combination ofPC and EMC.

[0084] For the supporting electrolyte, use is made of at least one ofLiClO₄, LiI, LiPF₆, LiAlCl₄, LiBF₄, CF₃, SO₃Li and so forth. Generally,the nonaqueous solvents stated above cannot fully release water and areapt to absorb water during the production of a battery. Therefore, thenonaqueous solvents often react with such a small amount of water tothereby produce hydrogen ions. Experiments showed that LiPF₆, and LiBF₄,in particular, noticeably produced hydrogen ions and tended to make theelectrolyte acid.

[0085] The present invention effectively takes up hydrogen ions and istherefore most advantageous when applied to a battery system using anelectrolyte that is apt to produce hydrogen ions. More specifically, thesupporting electrolyte should preferably be implemented by LiPF₆ orLiBF₄ and should preferably have a concentration of, e.g., 0.8 M to 1.5M.

[0086] As for a negative electrode active agent, use is made of lithium,lithium alloy, graphite capable of occluding and releasing lithium, oramorphous carbon or similar carbon.

[0087] The separator may be implemented by any one of woven cloth, glassfibers, porous synthetic resin film or the like. For example, a porousfilm of polypropylene or polyethylene is desirable from the standpointof thinness, broadness, strength, and resistance.

[0088] While the material of the casing also included in the presentinvention is not limited, it may be implemented by conventional resinfilms or laminate films by way of example. When the films are bonded byheat, it is preferable to use laminate films configured such that theirthermo-bonding films are positioned at the sealing side (generatingelement side). For example, use may be made of a trilayer laminate filmmade up of a heat-resistant resin film laminated on one surface of metalfoil and a thermo-bonding resin film laminated on the other surface ofthe metal foil. Alternatively, use may be made of a multilayer laminatefilm having a heat-resistant resin layer between metal foil and athermo-bonding resin or having an adhesive layer between metal andthermo-bonding resin.

[0089] The metal foil may be implemented by aluminum, copper, stainlesssteel, nickel, gold or silver by way of example. Among them, aluminum isparticularly desirable. For the heat-resistant resin film, use may bemade of polyethylene terephthalate or similar polyester or nylon by wayof example. The thermo-bonding resin film may be implemented by, e.g.,ionomer, polyethylene or polypropylene with or without maleic unhydrideor similar acidic group grafted thereto or copolymerized therewith.

[0090] For the films constituting the casing, use may alternatively bemade of films each consisting or a film of polyethylene terephthalate orsimilar heat-resistant resin and the previously mentioned thermo-bondingresin laminated on the film either directly or via adhesive. Further,each film may be implemented as a single film formed of thermo-bondingresin.

[0091] The illustrative embodiment will be described more specificallyin relation to specific estimation tests although the estimation testsare not limitable, but are only illustrative. In the estimation tests,the surface area and D₃₀ grain size were measured by Quanta Sorbavailable from Quanta Chrome Inc. and FRA available from Micro TracInc., respectively.

Estimation Test 1

[0092] To produce lithium manganate, lithium carbonate (Li₂CO₃) andelectrolytic manganese diode (EMD) were used as starting substances.Before the mixing of the two starting substances, Li₂CO₃ was pulverizedwhile EMD was sieved in order to promote reaction and to produce lithiummanganate having the target grain size. When lithium manganate isexpected to form part the material of the positive electrode of abattery, the weight mean particle size thereof should preferably be 5 μmto 30 μm in consideration of the balance between uniform reaction, easyproduction of slurry, safety, and so forth. In light of this, EMD, likelithium manganate, was provided with the grain size of 5 μm to 30 μm.

[0093] On the other hand, Li₂CO₃ should preferably have a grain size of5 μm or below in order to guarantee uniform reaction. For this reason,Li₂CO₃ was pulverized to a D₅₀ grain size of 1.4 μm.

[0094] EMD and Li₂O₃ provided with a particular grain size each weremixed such that the Li/Mn ratio was 1.05/2. The resulting mixture wassintered at 800° C. in an oxygen flow atmosphere. Subsequently, lithiummanganate particles whose particle size was 1 m or below were removed byan air sieve The lithium manganate particles left had a specific surfacearea of about 0.9m²/g. Further, the powder had a tap density of 2.17g/cc, a true density of 4.09 g/cc, a D₅₀ particle size of 17.2 μm, and alattice constant of 8.227 Å.

[0095] For the hydrogen ion uptaking agent, LiNi_(0.9)Co_(0.1)O₂ havinga specific surface area of 1.7 m²/g was prepared as an example of alithium-nickel compound oxide.

[0096] Assuming a weight ratio of 100— a:a, then lithium manganese andLiNi_(0.9)Co_(0.1)O₂ prepared as stated above were introduced such thata was 0 (Comparative Example), 1, 2, 3, 5, 10, 15 or 20. 5 g of each ofsuch powder mixtures was introduced in a solvent consisting of propylenecarbonate (PC) and dimethyl carbonate (DMC) (50:50 (volume %)) in orderto prepare an electrolyte. 100 cc of each electrolytes was introduced ina hermetic vessel. Such vessels each containing particular one of thesolvents were-heated to 800° C. and left for twenty days. Subsequently,the electrolytes ware produced from the vessels in order to analyze Mnion concentrations by ICP. FIG. 3 lists the results of analysis.

[0097] As FIG. 3 indicates, Mn was eluted more in the electrolyte as themixture ratio of LiNi_(0.9)Co_(0.1)O₂ increased, enhancing the hydrogenion uptaking effect. Presumably, therefore, the stability of thepositive electrode active substance increases even when a battery isused in a high-temperature environment.

Estimation Test 2

[0098] The hermetic vessels prepared in Estimation Test 1 were heated to80° C. and left for twenty days. Subsequently, the electrolytes wereproduced from the vessels in order to analyze Li ion concentrations byatomic absorption spectroscopy. FIG. 4 lists the results of analysisFIG. 4 suggests that a decrease in the Li concentration of theelectrolyte can be reduced if the mixture ratio of LiNi_(0.9)Co_(0.1)O₂is increased. This is because a mixture solvent of propylene carbonate(PC) and dimethyl carbonate (DMC) (50:50 (volume percent)) andcontaining LiPF₆ (concentration of 1 M) is about 6,400 ppm.

[0099] The results of Estimation Tests 1 and 2 indicate that thelithium-nickel compound oxide reduces the elution of Mn into theelectrolyte and therefore a change in Li ion concentration in theelectrolyte. Assume one-third of an Mn concentration or less without thelithium-nickel compound oxide and [lithium manganese compoundoxide]:[lithium-nickel compound oxide]=100:a (weight %) . Then, themixture ratio of the lithium-nickel compound oxide is a≧3. In addition,the results of Estimation Test 2 shows that when a≧3 holds, the Liconcentration in the electrolyte remains above 95% even when left fortwenty days at 80° C. The mixture ratio of a≧3 is therefore particularlydesirable.

Estimation Test 3

[0100] Lithium manganate produced in the same manner as in EstimationTest 1 was used as a lithium-manganese compound oxide. As for alithium-nickel compound oxide, a 2320 coil cell was produced by use ofLiNi_(0.8)Co_(0.2)O₂ having a specific surface area of 1.7 m²/g. As fora positive electrode, lithium manganese, LiNi_(0.8)Co_(0.2)O₂, aconducting agent and PTFE were mixed in a ratio of 72:8:10:10 (weight %)(a=10), kneaded, rolled to a thickness of 5 mm, and then punched. Theconducting agent was implemented by carbon black. A negative electrodewas formed of Li and provided with a diameter of 14 mm and a thicknessof 1.5 mm while a separator was implemented by a 25 μm thick, porous PPfilm. A mixture solvent of ethylene carbonate (EC) and dimethylcarbonate (DMC) (50:50 (volume %) and containing LiBF₄ (concentration of1 M) was used as an electrolyte.

[0101] For comparison, there was produced a coin cell identical with theabove coin cell except that its positive electrode contained lithiummanganate, a conducting agent and PTFE in a ratio of 80:10:10 (weight %)and did not contain LiNi_(0.8)Co_(0.2)O₂ (comparative example).

[0102] Charge-discharge cycle tests were conducted with the above twocoin cells under the following conditions. A constant current of 0.5mA/cm² was used for both of charging and discharging. Thecharge-discharge voltage ranged from 3.0 V to 4.5 V vs Li. Theestimation temperature was varied from 10° C. to 60° C. on a 10° C.basis. FIG. 5 lists #50/#1 residual capacity ratios (%) (ratios ofresidual capacity measured at the fiftieth cycle to residual capacitymeasured at the first cycle) with respect to various cycle estimationtemperatures . As FIG. 5 indicates, the coin cell of the example has ahigher residual capacity ratio than the comparative example even whenthe cycle temperature is raised.

Estimation Test 4

[0103] Lithium manganate produced in Estimation Test 1 was used as alithium-manganese compound oxide. As for a lithium nickel compoundoxide, use as made of LiNi_(0.8)Co_(0.2)O₂ having a specific surfacearea of 1.7 m₂/g. A casing was implemented by aluminum laminate films.

[0104] First, lithium manganese, LiNi_(0.8)Co_(0.2)O₂ and a conductingagent were mixed in a dry condition and then evenly dispersed in aN-methyl-2-pyroridone (NMP) in which PVDF was dissolved as a binder,thereby preparing slurry. The conducting agent was implemented by carbonblack. The slurry was coated on 25 μm thick aluminum foil, and then NMPwas evaporated to produce a positive electrode sheet. The solid ratio ofthe positive electrode sheet was lithiummanganese:LiNi_(0.8)Co_(0.2)O₂:conducting agent:PVDF=72:10:10 (weight %)(a=10) .

[0105] To produce a negative electrode sheet, carbon and PVDF were mixedin a ratio of 90:10 (weight %) and then dispersed in NMP. The resultingmixture was coated on 20 μm thick copper foil.

[0106] Subsequently, the positive and negative electrode sheets werewound round an oval core with the intermediary of a 25 μm thick, porousfilm separator formed of polyethylene. The resulting assembly wassubjected to heat pressing so as to produce a thin, oval electrodewinding.

[0107] Two pieces were cut away from a laminate film made up of 70 μmthick polypropylene resin (seal layer), 20 μm thick polyethyleneterephthalate, 50 μm thick aluminum and 20 μm thick polyethyleneterephthalate, which were sequentially laminated in this order. The twopieces each were formed with a recess including a bottom and sidescomplementary to the contour of the winding. The two pieces were thenpositioned face-to-face in such a manner as to embrace the winding andthen bonded by heat at their sides. As a result, the secondary batterywith the casing (1 a and 1 b) shown in FIGS. 1 and 2 were completed.Before the last side of the casing was bonded by heat, the winding wasimpregnated with an electrolyte. The winding with the electrolytecorresponds to the heat generating element 2, FIG. 2. The electrolytewas implemented by a mixture solvent of propylene carbonate (PC) anddiethyl carbonate (LEC) (50:50 (weight %)) The last side was sealed in adepressurized condition.

[0108] For comparison, there was produced a secondary battery with acasing identical with the above battery except that its positiveelectrode did not contain LiNi_(0.8)Co_(0.2)O₂, and that the solid ratiowas lithium manganate:conducting agent:PVDF=80:10::10 (weight %)(comparative example).

[0109] Charge-discharge cycle tests were conducted with the twosecondary batteries under the following conditions. Charging waseffected with a current of 500 mA and a voltage of up to 4.2 V whiledischarging was effected with a current of 1,000 mA and a voltage of upto 3.0 V. FIG. 6 compares the cycle characteristics of the 55° C.discharge capacity particular to the example and comparative example. Itwill be seen that the deterioration of capacity is smaller in theexample of the illustrative embodiment than in the comparative exampleafter the repeated charge-discharge cycle.

Estimation Test 5

[0110] Lithium manganate produced in the same manner as Estimation Test1 was used as a lithium-manganese compound oxide. As for alithium-nickel compound oxide or hydrogen ion uptaking agent, use wasmade of LiNi_(0.8)cO_(0.15)Al_(0.05)O₂ having a specific surface area of1.7 m²/g was used.

[0111] First, lithium manganate, LiNi_(0.8)Co_(0.15)Al_(0.05)O₂ and aconducting agent was mixed in a dry condition and then evenly dispersedin a binder, thereby preparing slurry. The binder was implemented byN-methyl-2-pyroridone (NMP) in which PVDF was dissolved. The slurry wascoated on a 25 μm thick aluminum foil, and then NMP was evaporated toproduce a positive electrode sheet. The positive electrode sheet had asolid ratio of lithium manganate:LiNi _(0.8)Co_(0.15)Al_(0.05)O₂:conduction agent:PVDF=80−X:X:10:10 (weight %). FIG.7 lists various values of X (weight %) used for tests. FIG. 7 shows theresults of tests together with the values of a (=X 100/80; identicalmeaning with the previous a). A comparative example shown in FIG. 7corresponds to x=0 (a=0).

[0112] To produce a negative electrode sheet, carbon and PVDF were mixedin a ratio of 90:10 (weight %), dispersed in NMP, and then coated on 20μm thick copper foil.

[0113] An electrolyte was implemented by a mixture solvent of propylenecarbonate (PC) and diethyl carbonate (DEC) (50:50 (weight %)) andcontaining a supporting electrolyte implemented by 1 M of LiPF₆. For aseparator, use was made of 25 m thick, porous film formed ofpolyethylene.

[0114] Capacity preservation tests were conducted with the secondarybatteries produced by the above procedure. Charging was effected with aconstant current of 500 mA up to a voltage of 4.2 V and was theneffected with a constant voltage of 4.2 V for 2 hours. Subsequently,discharge capacity was measured in a condition wherein the batterieswere immediately caused to discharge at room temperature and a conditionwherein they were left at room temperature for twenty-eight days andthen caused to discharge. For the measurement, a current of 500 mA and acutoff voltage of 3.0 V were used in a room temperature environment.

[0115]FIG. 7 lists capacities measured after twenty-eight days (labeled4W Capacity) and the ratios thereof to capacities immediately measured(labeled 4W Capacity). It will be seen that the example of theembodiment preserves a greater capacity than the comparative exampleeven when left for twenty-eight days. In addition, the high capacity,lithium-nickel compound oxide successfully increases the capacity of thesecondary battery.

Estimation Test 6

[0116] Safety tests were conducted with the secondary battery producedin Estimation Test 5. FIG. 8 shows the results of the safety tests. Whenthe positive electrode active agent is implemented mainly by lithiummanganate, differences are difficult to see as to a short-circuit test,a hot box and other safety estimation items because lithium manganate ishigher in safety than Co. To make the above differences conspicuous,secondary batteries were produced with a positive electrode density ashigh as 3.1 g/cm³. Such a severe condition is important in view of thefuture tendency toward higher capacity. For the estimation of safety, anovercharge test and a nail drive test were used. The overcharge test wasconducted under the conditions of 12 V and 3C. The nail drive test wassuch that a nail was driven into a battery so as to forcibly short it.The nail drive test was effected in accordance with UL-1642 by using 4mm nails.

[0117] As for the overcharge test, no smoke or fire was observed evenwhen x was 56 or above. As for the nail drive test, some vapor or firewas observed when x was 40 or above, From the safety standpoint,therefore, x should preferably be 36 or below while a should preferablybe smaller than equal to 45.

Estimation Test 7

[0118] Lithium manganate produced in the same manner as in EstimationTest 1 and LiNi_(0.8)Co_(0.1)Mn_(0.1) 0 ₂, which is a specificlithium-nickel compound oxide, were mixed. Again, assuming a mixtureratio of 100−a:a (weight %), a was selected to be 0 (comparativeexample), 3, 5, 10, 15, 20, 30 and 35. Electrolytes each consisting ofethylene carbonate (EC) and diethyl carbonate (DEC) (50:50 (weight %)and containing 5 g of particular one of the above mixtures and 10 cc ofLiPF₆ (concentration of 1 M) were put in hermetic vessels. There wereused seven different kinds of LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ respectivelyhaving specific surface areas of 3.0 m²/g, 2.36 m²/g, 1.50 m²/g, 0.71m²/g, 0.49 m²/g, 0.30 m²/g and 0.25 m²/g.

[0119] The hermetic vessels were heated to 80° C. and left for twentydays. Subsequently, the electrolytes were produced form the vessels inorder to analyze Mn ion concentrations by ICP. FIG. 9 shows the resultsof analysis. As FIG. 9 indicates, the elution of Mn into the electrolyteis more reduced as the specific surface area increase.

[0120] For the safety purpose, a included in the ratio of (lithiummanganate):(lithium-nickel compound oxide)=100−a:a (weight %) shouldpreferably be smaller than or equal to 45, as stated earlier. On theother hand, FIG. 9 indicates that for the specific surface area is 0.25m²/g the amount of Mn to be eluted cannot be reduced to less thanone-third of the amount of Mn elution of 2,320 ppm, which occurs withoutthe lithium-nickel compound oxide, unless the mixture ratio of thelithium-nickel compound dioxide is increased to 50%. It is thereforepreferable that the lithium-nickel compound oxide has a specific surfacearea X greater than 0.3 m²/g.

Estimation Test 8

[0121] Lithium manganate produced in the same manner as in EstimationTest 1 was used as a lithium-manganese compound oxide. As for alithium-nickel compound oxide, there were prepared five different kindsof LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ powder respectively having specificsurface areas of 4.5 m²/g, 3.2 m²/g, 3.0 m³/g, 1.50 m²/g and 0.30 m²/g.The above Lithium manganate and LiNi _(0.8) CO_(0.1)Mn_(0.1)O₂ andcarbon black, which is a specific conducting agent, were mixed in a drycondition. The resulting mixture was added to a binderN-methyl-2-pyroridone (NMP), in which PVDF was dissolved, and thenevenly dispersed by kneading so as to produce slurry. The mixture ratioof lithium manganese, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂,conducting agent,PVDF and NMP was 30:10:5:5:50 (weight %)

[0122] After the measurement of viscosity using a Brookfield viscometer,the slurry was uniformly coated on 25 μm thick aluminum foil. NMP wasthen evaporated to complete a positive electrode sheet. FIG. 10 shows arelation between the specific surface area of the lithium-nickelcompound oxide and the viscosity and state or the slurry and the coatedcondition of the electrode. It will be seen that when the specificsurface area is 3.2 m²/g or above, the slurry gels and is difficult tocoat the electrode. Therefore, the specific surface area of thelithium-nickel compound oxide should preferably be 3.0 m²/g or below.Also, to reduce the elution of Mn into the electrolyte, the D₅₀ grainsize should preferably be 40 μm or below and, to promote easy coating, 3μm or above.

[0123] To summarize the results of Estimation Test 8, the optimalspecific surface area X of the lithium-nickel compound oxide to be mixedlies in the range of 3≦X≦3.0 (m²/g) from the Mn elution standpoint andfrom the slurry coating and printing standpoint. Also, as for thelithium-nickel compound oxide to be mixed, the optimal D₅₀ grain sizeshould be 3 μm or above, but 40 μm or below from the Mn dilutionstandpoint and the slurry coating and printing standpoint. Further,assuming that [LiMn compound oxide]:[LiNi compound oxide] is 100−a:a,then a should preferably lie in the range of 3≦a≦45 from the Mn dilutionstandpoint and safety standpoint.

Estimation Test 9

[0124] The initial thickness of the battery of the illustrativeembodiment produced in Estimation Test 4 and that of the battery of thecomparative example were measured. Also, the thickness was measuredafter 100 cycles of charge-discharge cycle test effected at 55° C.Charging was effected with a current of 500 mA and up to a voltage of4.2 V while discharging was effected with a current of 1,000 mA, and upto a voltage of 3.0 V. FIG. 11 shows the results of measurement. It willbe seen that the battery of the illustrative embodiment has its contourswelled less than the contour of the battery of the comparative exampleafter the charge-discharge cycles.

Estimation Test 10

[0125] How the battery of the illustrative embodiment and that of thecomparative example swell was determined by holding then at 60° C.First, the initial thickness of the two batteries was measured. Thebatteries were then charged by a constant current of 500 mA up to avoltage of 4.2 V and then charged by a constant voltage of 4.2 V for 2hours. Thereafter, the batteries were left at 60° C. for twenty-eightdays. The resulting thickness of each battery is shown in FIG. 12. Itwill be seen that the battery of the illustrative embodiment swells lessthan the battery of the comparative example.

[0126] In summary, in accordance with the present invention, a secondarybattery is capable of effectively reducing a hydrogen ion concentrationin an electrolyte and therefore reducing the elution of Mn from alithium-manganese oxide, which is a positive electrode active substance,as well as a change in the Li concentration in the electrolyte. Thissuccessfully improves a charge-discharge cycle, particularly acharge-discharge life at high temperatures, and a capacity preservationcharacteristic. In addition, the battery swells little despite thecharge-discharge cycle or storage.

[0127] Various modifications will become possible for those skilled inthe art after receiving the teachings of the present disclosure withoutdeparting from the scope thereof.

What is claimed is:
 1. In a nonaqueous electrolyte type secondarybattery comprising an electricity generating element, which includes atleast appositive electrode implemented by a lithium-manganese compoundoxide, a negative electrode, an electrolyte and a separator, and filmsencasing said electricity generating element, a composition causing saidelectrolyte to react with water to thereby produce hydrogen ions isprovided, and a hydrogen ion uptaking agent is so positioned as tocontact said electrolyte existing in said secondary battery.
 2. Asecondary battery as claimed in claim 1 , wherein the electrolytecontains a supporting electrolyte comprising either one or LiPF₆ andLiBF₄.
 3. A secondary battery as claimed in claim 2 , wherein thehydrogen ion uptaking agent is mixed in the positive electrode or mixed,dissolved or dispersed in the electrolyte.
 4. A secondary battery asclaimed in claim 3 , wherein the hydrogen uptaking agent bifunctions asa positive electrode active substance and is mixed in the positiveelectrode together with the lithium-manganese compound oxide.
 5. Asecondary battery as claimed in claim 4 , wherein the hydrogen ionuptaking agent comprises a lithium-nickel compound oxide having ahydrogen ion uptaking function.
 6. A secondary battery as claimed inclaim 5 , wherein assuming that a ratio in weight percent is[lithium-manganese compound oxide]:[lithium-nickel compoundoxide]=100−a:a, then a lies in a range of 3≦a≦45.
 7. A secondary batteryas claimed in claim 5 , wherein the lithium-nickel compound oxide has aspecific surface area X lying in a range of 3≦X≦3.0 (m²/g).
 8. Asecondary battery as claimed in claim 7 , wherein assuming that a ratioin weight percent is [lithium-manganese compound oxide]:[lithium-nickelcompound oxide]=100−a:a, then a lies in a range of 3≦a≦45.
 9. Asecondary battery as claimed in claim 5 , wherein the lithium-nickelcompound oxide has a D₅₀ particle size of 40 μm or below, but 3 μm orabove.
 10. A secondary battery as claimed in claim 9 , wherein assumingthat a ratio in weight percent is [lithium-manganese compoundoxide]:[lithium-nickel compound oxide]=100−a:a, then a lies in a rangeof 3≦a≦45.
 11. A secondary battery as claimed in claim 1 , wherein thehydrogen ion uptaking agent is mixed in the positive electrode or mixed,dissolved or dispersed in the electrolyte.
 12. A secondary battery asclaimed in claim 11 , wherein the hydrogen uptaking agent bifunctions asa positive electrode active substance and is mixed in the positiveelectrode together with the lithium-manganese compound oxide.
 13. Asecondary battery as claimed in claim 12 , wherein the hydrogen ionuptaking agent comprises a lithium-nickel compound oxide having ahydrogen ion uptaking function.
 14. A secondary battery as claimed inclaim 13 , wherein assuming that a ratio in weight percent is[lithium-manganese compound oxide]:[lithium-nickel compoundoxide]=100−a:a, then a lies in a range of 3≦a≦45.
 15. A secondarybattery as claimed in claim 13 , wherein the lithium-nickel compoundoxide has a specific surface area X lying in a range of 3≦X≦3.0 (m²/g).16. A secondary battery as claimed in claim 15 , wherein assuming that aratio in weight percent is [lithium-manganese compoundoxide]:[lithium-nickel compound oxide]=100−a:a, then a lies in a rangeof 3≦a≦45.
 17. A secondary battery as claimed in claim 13 , wherein thelithium-nickel compound oxide has a D₅₀ particle size of 40 μm or below,but 3 μm or above.
 18. A secondary battery as claimed in claim 17 ,wherein assuming that a ratio in weight percent is [lithium-manganesecompound oxide]:[lithium-nickel compound oxide]32 100−a:a, then a liesin a range of 3≦a≦45.